This invention relates to an arrangement for determining a medium in general, and, more particularly, to an apparatus and method for determining the properties of a medium, especially a flowing medium.
The known arrangements of radiation detectors with temperature sensors for determining radiation, while having otherwise good properties, such as spectral range sensitivity and ruggedness, have response times which are too slow for such measurements. This is particularly true for the common techniques which use alternating light or filter wheel modulation.
In a known method of monitoring the flow of a medium, a thermistor is adjusted by means of a passing electric current to maintain a working temperature which is above the ambient temperature. A flow stream of the medium is passed over the thermistor. Through a differential amplifier circuit, having a feedback branch, the cooling caused by the flow of the medium with the resulting resistance drop of the thermistor is compensated for by varying the current so that the temperature of the thermistor is kept at a constant value at the working temperature. The power necessary to maintain the constant working temperature is dependent on the flow of the medium and is monitored by means of an indicator. A disadvantage of this method is that high temperatures in the range of about 200.degree. C. are required in order to shorten the reaction time of the thermistor, thereby limiting the range of application and requiring protective measures. (See Swiss Pat. No. 549,218).